Carbon fiber-reinforced gypsum models and forming molds

ABSTRACT

Gypsum models and molds made of a gypsum structure uniformly dispersing therein carbon fibers having a predetermined length in a defined range of amount are described. The molds include various types of molds such as original mold, basic mold, forming mold and the like. A method for manufacturing these molds is also described in which sizing agents deposited on carbon fibers are decomposed and removed by heating or dissolution in solvent, separating the carbon fibers into single fibers in water, adding a major part of gypsum powder to the carbon fiber dispersed water to obtain a gypsum slurry dispersing uniformly those single fibers, and casting the slurry in a case mold to obtain forming molds. A method for manufacturing white wares is also described in which a pottery or porcelain slip is casted into the gypsum forming molds, removing the resulting green product from the mold after hardening and drying the green products. Gypsum powder materials comprising carbon fibers and a method for making the same are also described.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to gypsum models, original molds and formingmolds, particularly gypsum molds for white wares or so-called plastermolds, which have an improved higher strength by dispersing areinforcing material in gypsum structure. It also relates to gypsumpowder materials for such models and molds and a method for producingthe models and molds and the powder materials.

2. Description of the Prior Art

Various methods of increasing the strength of gypsum models, originalmolds and forming molds for general forming purpose (hereinafter, thesemolds are referred to simply as gypsum mold) have been known. One suchmethod comprises mixing a major part of β-hemihydrate gypsum with aminor part of β-hemihydrate gypsum so as to reduce a water part, togypsum materials. It will be noted here that hemihydrate gypsum may alsobe called plaster of Paris. Another method is known in which gypsum isadmixed with cements or resins. In a further method, gypsum is admixedwith natural fibers such as hemp, or glass fibers.

However, the first-mentioned method in which β-hemihydrate gypsum isadmised with β-hemihydrate gypsum so as to reduce the water part isdisadvantageous in that although the strength itself is slightlyimproved, the water absorbability which is essential for forming whitewares lowers. Good water absorbability of gypsum molds particularly atthe time of slip casting is one of the most important factors requiredfor gypsum forming mold. If the water absorbability of a gypsum mold ispoor, a forming time for one product is prolonged, thus lowering theforming productivity. Especially, when the water absorbability of theslip casting molds at the time of slip casting is poor, the resultinggreen products is ill-shaped. Thus, the lowering of the waterabsorbability in a gypsum forming mold is a vital drawback.

In the case where gypsum is admixed with cements or resins, the strengthof the molds or models may be improved with an attendant disadvantagethat the water absorbability lowers and the other their propertieschanges seriously. With the case of mixing gypsum with natural fiberssuch as hemp, natural fibers have so small a tensile strength ascompared with synthetic fibers that an increase of the strength cannotbe attained unless large amounts of the fibers are mixed to gypsum. Theuse of natural fibers in an increased amount accrues to a lowering ofthe water absorbability of the resulting gypsum mold. In addition, thesingle fiber of the natural fibers are so thick that the fibersincorporated in the surface portion of the gypsum mold are liable toexpose their end portion at the forming faces. The exposed fiber ends atthe forming faces may damage the surfaces of a forming products such asgreen ware and the portions where the fiber ends are exposed will losewater absorbability of the mold. Thus, the surface properties of theportions result in being uneven. In the case of the forming mold, theforming failure tends to occur because of this uneven surface.

The method of mixing glass fibers with gypsum will slightly improve thestrength of the gypsum mold. However, when the ends of the glass fibersincorporating in the surface portions are exposed on forming faces,because of the rigidity of glass fibers, the surface of the formingproducts such as green ware may be damaged.

SUMMARY OF THE INVENTION

Broadly, the present invention provides a gypsum model i.e. patterns andmold which is made of a uniform mix of gypsum and a predetermined amountof carbon fibers with a predetermined length and a very small diameterwhich have excellent strength characteristics, flexibility, lightweight, and low thermal expansion coefficient. By uniformly dispersing arelative small amount of carbon fibers into the gypsum structure,mechanical or physical strength of material dynamics and thermaldurability of the model and mold are improved without a lowering ofwater absorbability, one of the important physical properties of formingmolds for making white wares and paper wares, also without lowering ortransforming other physical properties of models and molds for the othergeneral purpose. In other words, the use of carbon fibers in combinationwith gypsum contributes to increase the strength of material mechanics,resulting in improving both a strength against external mechanical force(mechanical durability) and a strength against internal stress caused bythermal strain due to uneven temperature distribution in the products(thermal stability or durability).

It is accordingly an object of the present invention to provide gypsummodels and molds of the just-mentioned types whereby without loweringvarious inherent properties of gypsum molds, the strength of materialdynamics of the gypsum mold are increased, i.e. both a strength againstexternal mechanical force and a strength against internal stress causedby thermal strain are improved at the same time. Consequently, breakageof the forming molds due to an external pressure or force imposed at thetime of forming is not able to occur and, in the case of the formingusing gypsum absorbability, for instance, the forming of the white wareand the paper ware, the gypsum forming molds can be dried at highertemperatures after every cycle of forming operations, enabling one toshorten the drying time resulting in improved forming ability per mold(productivity by the gypsum forming mold).

It is another object of the invention to provide a method for making agypsum model and mold comprising carbon fibers as a reinforcement whichcomprises dispersing uniformly a multitude of single carbon fibers in agypsum slurry, whereby the carbon fibers are uniformly dispersed in thestructure of the gypsum model and mold.

It is a further object of the invention to provide a gypsum powdermaterial, comprising a gypsum powder admixing uniformly with singlecarbon fibers in a predetermined mixing ratio.

Other objects and advantages and features of the present invention willbecome apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the length of carbonfibers to be admised with gypsum and the bending strength of the gypsumtest pieces;

FIG. 2 is a graph showing the relation between weight percent of thecarbon fibers content based on gypsum powder and the bending strength ofthe gypsum test pieces;

FIG. 3 is a graph showing the relation between the weight percent ofcarbon fibers content based on gypsum powder and water absorbability(weight percent of water to be able to be absorbed in the gypsumstructure, to gypsum) of the gypsum products;

FIG. 4 is a schematic perspective view of a container for making agypsum slurry;

FIG. 5 is a schematic sectional view showing a forming step of making agypsum mold for jiggering a flat white ware adapted to form an innerperipheral surface or portion of a dish ware;

FIG. 6 is a schematic sectional view showing, partially broken, thegypsum forming mold of FIG. 5 for jiggering the flat white ware;

FIG. 7 is a schematic sectional view showing a forming step of making agypsum mold for jiggering a flat white ware adapted to form an innerperipheral surface of a dish ware in which the outer peripheral surfaceof the mold is covered with a thin layer of pure gypsum;

FIG. 8 is a schematic perspective view, partially broken, showing thegypsum forming mold of FIG. 7;

FIG. 9 is a schematic sectional view showing a forming step of making agypsum mold for jiggering a hollow white ware adapted to form an outerperipheral surface of a cup ware in which the inner peripheral surfaceof the mold is covered with a thin layer of pure gypsum;

FIG. 10 is a schematic sectional view of the gypsum forming mold of FIG.9;

FIGS. 11(a) through 11(d) are schematic sectional views showing aprocess of manufacturing the gypsum molds for slip casting of theporcelain relief plate, including a model or an original mold in FIG.11(a), the basic mold in FIG. 11(b), the case mold in FIG. 11(c) whichis a female mold for the forming mold, and the forming mold for slipcasting in FIG. 11(d) (these four types of the mold are called simply asthe gypsum mold);

FIG. 12 is a schematic view showing the manner of mixing by movement ofrotating and rocking;

FIG. 13 is a graph showing the relation between bending force and thetime under the certain test condition, i.e. the certain weight percentof carbon fibers in the gypsum structure as a parameter;

FIG. 14 is a picture of the gypsum mold composition in the case of usingβ-hemihydrate gypsum by scanning type electron microscope;

FIG. 15 is a picture of the same except using β-hemihydrate gypsum asFIG. 14; and

FIG. 16 is a graph showing the relation between expansion ratio, i.e.,variation ratio in length, and temperature for a pure gypsum test pieceand a carbon fiber-reinforced gypsum test piece, respectively.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS OF THE INVENTION

Original gypsum molds or models in the present invention may include allgypsum models which are used in processing metals, non-metals orinorganic or organic materials by press work, forging, casting, cutting,grinding, laser work, and other various chemical and physical workings.

Biological or medical models such as internal organ which have norelations to processing of material, are also included.

Gypsum molds for forming to which the present invention is directed arealmost all molds for forming and include not only gypsum molds forcasting, especially precision casting or die casting press forming frompowders and other plastic materials, extrusion forming, injectionforming, but also gypsum molds for other plastic formings. Moreover,gypsum molds for forming are almost all molds for forming ceramics andinclude not only gypsum molds for forming, for example jiggering andcasting, of white wares, porcelains and pottery, and refractoryarticles, but also gypsum molds for press forming, injection forming,slip casting, and the like formings of so-called fine ceramics, whichare free of any clays upon preparation of starting materials, such asalumina, silicon carbide, silicon nitride, partially stabilizedzirconia, Sialon and the like.

Also, the gypsum molds include all molds which are fabricated in aprocess on the way of manufacturing forming molds from original molds.Typicals of such molds are a basic mold and a case mold in themanufacture of a mold for forming white ware.

Carbon fibers used in the practice of the present invention may be madefrom polyacrylonitrile, pitches, rayons, and/or lignin-poval materials.In order to impart high or increased strength to gypsum molds, highstrength or high elasticity carbon fibers are desirable. Moreparticularly, carbon fibers have preferably a tensile strength not lessthan 200 kgf/mm² (kg/mm²) and a tensile modulus not less than 20,000kgf/mm² (kg/mm²).

In the present invention, it is very important that carbon fibers whichare apt to form lumps be uniformly dispersed into a gypsum slurrywithout producing lumps and be uniformly dispersed in gypsum structureto form a uniform matrix. To this end, carbon fibers incorporated intogypsum as a reinforcing material should have a predetermined length anda predetermined content to gypsum.

Carbon fibers are dispersed in the gypsum matrix in the form of singlefibers. For the reasons described hereinafter, carbon fibers should havea length ranging from 5 to 100 mm, preferably from 20 to 30 mm.

Referring now to the accompanying drawings and particularly to FIG. 1.FIG. 1 shows test results showing the relation between bending strength(modulus of rupture) of a 15 mm×25 mm×250 mm gypsum test piece andlength of carbon fibers. The test piece is made of a gypsum compositionmade of 100 parts by weight of gypsum, 60 parts by weight of water and0.5 part by weight of carbon fibers. As will be clearly seen from thisfigure, the bending strength is abruptly lowered when the length isbelow 15 mm. The reason why the length of carbon fibers dispersed in thestructure of the gypsum mold is determined in the range of from 5 to 100mm is as follows: when the length is below 5 mm, the resulting gypsummold is not satisfactorily improved in strength because of theinsufficiency of total adhesion or contacting area between gypsumparticles and single carbon fibers in the mold structure. On the otherhand, lengths larger than 100 mm are disadvantageous in handling at thetime when carbon fibers are separated into single fibers, when gypsumpowder and water are mixed and agitated, and when the slurry is pouredinto a case mold. Consequently, uniform dispersion of carbon fibers intogypsum matrix is difficult.

The single carbon fibers for the purpose of the invention are obtainedby the following steps. Firstly the carbon fibers produced by ordinarytechniques, are cut into pieces having a length of from 5 to 100 mm, andthen separated into a multitude of single fibers according to thefollowing methods.

One such method of separating bundles of carbon fibers into singlefibers is a method in which sizing agent applied onto the surfaces of asingle fiber on the purpose to facilitate handling of the fibers, isremoved, followed by dispersing them in water with supersonic agitation.For removing the sizing agent, the bundles of carbon fibers are heatedin oxidizing atmosphere to oxidize and decompose or are washed withacetone solvent to dissolve and remove the sizing agent therefrom.Removal of the sizing agent by heating is carried out at temperaturesdepending on the type of sizing agent applied onto the surfaces.Conveniently, the heating temperature is about 300° C. which is amaximum temperature at which a majority of carbon fibers may be safelytreated. The carbon fibers removed the sizing agent by heating arelikely to be separated into a multitude of single fibers (usually 1,000to 24,000 fibers) of a very small diameter (usually 5 to 10 microns).

The carbon fibers from which the sizing agent has been removed and whichhave been cut into pieces having a size of from 5 to 100 mm are chargedinto a water vessel and gently agitated by agitator blades applyingultrasonic oscillation. As the carbon fibers removed the sizing agentare easily separated in water, they disperse in water by the synergisticeffect of the ultrasonic oscillation and the gentle agitation, into amultitude of single fibers having a very small diameter withoutproducing any lumps. In order to avoid the carbon fibers from beingdamaged by means of the agitator blades in agitating, the agitatingshould be effected at 40 to 60 r.p.m. when the water vessel has adiameter of about 60 cm. After the dispersion, the separated singlefibers are collected from the water vessel, followed by removing watertherefrom and drying.

An alternative method of separating bundles of carbon fibers into singlefibers is a method of using the carbon fibers treated with awater-soluble sizing agent. More particularly, bundles of carbon fiberswhich have been treated with a watersoluble sizing agent are first cutinto pieces having a length ranging from 5 to 100 mm and weighed to havea predetermined amount of carbon fibers required for one batch. Theweighed carbon fibers are charged into a container 1, as shown in FIG.4, having a predetermined amount therein for one batch. Then, thedispersion is gently agitated by means of rotating agitator bladeswhereupon the water-soluble sizing agent applied onto the carbon fibersare immediately dissolved in water and self-diffused. By the agitationwith the impeller, thus, the bundles of carbon fibers are uniformlydispersed and separated into a multitude of single fibers with a verysmall diameter without producing any lumps. In this method also, thecarbon fibers are prevented from being damaged by the agitator bladeswhen the agitation is effected at 40 to 60 r.p.m. in the case ofcontainer having a diameter of about 60 cm. Thus, carbon fibers areseparated into single fibers by this method, and a uniform dispersion ofsingle carbon fibers in water is obtained in the container 1 as shown inFIG. 4. In this case, the dispersion may be subsequently admixed in thecontainer 1 with a gypsum powder and additives such as a hardeningretarder, i.e. curing retarder, a dehydrating agent, i.e. water reducerand the like in amounts sufficient for the batch, followed by agitatingto obtain a gypsum slurry or slip.

For the production of a gypsum slurry having single carbon fibersdispersed or incorporated therein, in the above each method, the contentof carbon fibers based on gypsum dry powder is in the range of from 0.01to 1 wt % (about 0.008 to 0.9 wt % when calculated as a ratio of carbonfibers to the gypsum in cured gypsum mold), preferably from 0.1 to 0.3wt %.

FIG. 2 shows the relation between the bending strength of a gypsum testpiece and the content by wt % of the carbon fibers based on dry gypsumpowder. The gypsym test piece is made of a composition comprising 100parts by weight of gypsum powder, 60 parts by weight of water and thevarious amounts (parts by weight or weight percent based on gypsum drypowder) of the carbon fibers having a length of 20 mm and has adimension of 15 mm×25 mm×250 mm. In FIG. 3, there is shown the relationbetween the water absorbability (weight % of the maximum absorbablewater of the cured gypsum test piece) and the contents by wt % of thecarbon fibers based on dry gypsum powder. As will be clear from thefigures, the higher contents of carbon fibers result in the largerbending strength with the higher water absorbability.

The reason why the water absorability of the gypsum incorporated withcarbon fibers increases is considered due to the fact: particles ofgypsum have a needle-like shape and carbon fibers have a round orsimilar section; the size of gypsum particles is almost the same levelas the diameter of carbon fibers; and hence gaps or voids are newlyestablished among each carbon fibers and gypsum particles.

As indicated before, the content of carbon fibers in gypsum powder isdefined to be in the range of from 0.01 to 1 wt %. Amounts less than0.01 wt % are unfavorable because the ratio of the amounts of carbonfibers to gypsum powder is so low that the strength of the resultinggypsum mold is not satisfactorily increased. On the contrary, amountsmore than 1 wt % are also unfavorable for the following reason. Inpreparing a gypsum slurry, carbon fibers are so much in amount that theycannot be uniformly dispersed in the slurry and tend to form lumps ofthe carbon fibers. Such slurry shows poor fluidity at the time ofcasting or pouring into a case mold and makes it difficult to handle.Moreover, physical properties such as water absorbability or capillarityof the gypsum mold obtained from the slurry change and, therefore, donot satisfy the requirements as the mold for forming white wares. Alsoin the cured gypsum mold, lumps of the carbon fibers tend to appear andthey decrease the function as fine models or molds.

In order to suitably make a gypsum slurry uniformly dispersing thereincarbon fibers from which any sizing agent has been removed as describedabove in the first, predetermined amounts of water and necessaryadditives such as a hardening retarder (curing retarder), a dehydratingagent (water reducer) and the like required for one batch, are firstlycharged in the container 1 as shown in FIG. 4. Into the container aresecondly introduced a predetermined amount of previously weighed singlecarbon fibers and finally a predetermined amount of gypsum powder.Subsequently, the container 1 is set on a vacuum agitator and theagitator blades are rotated at a low speed. Thus, a gypsum slurry inwhich the single carbon fibers uniformly disperse without forming lumpsis obtained. The reason why carbon fibers are uniformly dispersed in thegypsum slurry without forming lumps is due to the fact that the ratio ofamount of the carbon fibers to the gypsum is very small.

For manufacturing gypsum molds for forming white wares, the gypsumslurry is then gently poured into a case mold 2 as shown in FIG. 5 andallowed to cure for predetermined time. After hardening, the case mold 2is applied with a slight shock to be separated into two pieces, theupper and the lower, from which the resulting forming molds are removed,followed by sufficiently drying at a predetermined temperature. As aresult, a gypsum mold 3 for jiggering a flat white ware, as shown inFIG. 6, which is adapted to jigger a dish is obtained. Carbon fibershave high flexibility and may be arbitrarily deformed after casting, sothat they are considered to be naturally forced into the void gap amongthe particles of gypsum. Carbon fibers in the surface portions of thegypsum mold rarely expose their end portion to the surfaces of the mold.Even though carbon fibers externally exposed, they have such a smalldiameter and high flexibility that do not damage the surfaces of a greenware product therewith.

As described above, even though ends of carbon fibers in the gypsum moldare exposed on the surfaces of the mold, a green product suffers littledamage on the surfaces thereof. In case it is necessary to obtain a verysmooth green surface as white ware products of high quality, the casemold 2 is firstly formed with a 1 to 3 mm thick thin layer 4 of puregypsum on its surface, as shown in FIG. 7. While pouring a small amountof the slurry of pure gypsum that does not contain any carbon fibers,the case mold 2 rotates at a low speed. Thereafter, the rotation of thecase mold 2 is stopped. Then immediately, the gypsum slurry in whichcarbon fibers are uniformly dispersed is gently cast into the case mold2 in the procedure as described with reference to FIG. 5. As a result ofthese two steps operation, a gypsum mold 5, as shown in FIG. 8, forjiggering of a flat white ware is obtained. This mold has the thin andfine layer 4 on the outer surface thereof which is a forming surface. Bythis, the exposed carbon fibers as described with reference to FIG. 5,are completely covered with the thin layer 4 and could not injure thefine surface of a green flat ware.

The gypsum mold 5 for flat ware jiggering is used to form an innersurface of a green ware product and is covered with the thin layer 4 ofpure gypsum in order to prevent exposure of carbon fibers on the formingsurface. In case of a gypsum mold for jiggering a hollow white ware,which is used to form an outer surface of a green ware, as shown in FIG.9, also following two steps operations are applied. Firstly, a slurry ofpure gypsum is poured into a case mold 6 while rotating the mold 6 at alow speed. A 1 to 3 mm thick thin layer 7 of pure gypsum is formed on anouter peripheral surface of a forming portion 6a (jiggering surface) ofthe case mold 6. Then, the case mold 6 is stopped rotating and a gypsumslurry uniformly dispersing therein carbon fibers is cast into the mold,followed by the process of curing, removing this forming mold from thecase mold and drying the forming mold, as previously described withreference to FIG. 5. In this manner, the gypsum mold 8 for jiggering ahollow white ware, having the thin layer 7 of pure gypsum on the innersurface thereof as shown in FIG. 10, is obtained and is adapted tojigger a cup.

For the manufacture of a gypsum mold for slip casting a white ware platewith a relief pattern, a procedure consisting of four types of gypsummolds shown in FIGS. 11(a) through 11(d) is used. In FIGS. 11(a) through11(d), an original mold or model 10 in FIG. 11(a), is firstly made. Theoriginal mold is made of a gypsum having the carbon fibers uniformlydispersed or an incorporated gypsum composition which is first made, theinternal parts of the carbon fibers uniformly dispersed gypsum andsecond the outer surface of a pure gypsum. Secondly, a basic mold 11 asshown in FIG. 11(b) is duplicated from the original mold or would bereproduced after this original mold will not have been used.

The basic mold 11 is a female mold of the original mold and is alsocalled "a master mold" in the case of precision metal casting. Thirdly,a gypsum female mold of the entire type of the basic mold 11 isfabricated and separated into the upper and the lower halves. As aresult of separating, a female mold 12 having the upper male portion anda female mold 13 of the lower portion are obtained. The lower femalemold 13 is designed to have a gypsum slurry inlet 14. The upper andlower female molds 12, 13 are combined to give a so-called case mold 15.Forthly, into the case mold 15, the gypsum slurry contained uniformlydispersed carbon fibers is cast from the inlet 14. When the charging ofthe gypsum slurry is effected at a normal or atmospheric pressure, thecase mold 15 as shown in FIG. 11(c) has to be reversed. After completionof the casting, the slurry is allowed to cure for a certain time andhardened, after which the upper female mold 12 and the lower female mold13 are separated from each other to obtain a forming mold 16. The carbonfiber-reinforced gypsum mold of the present invention has a reducedvolume expansion in curing and also, a reduced thermal expansioncoefficient at the end of curing. Therefore, in the whole process of thethree duplicating production steps, it is possible to produce andduplicate the pattern of the molds with high accuracy, and with analmost no variation in dimension during these processes producing theforming mold from the original mold, if the carbon fiber-containinggypsum is used. This is also true of manufacturing the case molds forjiggering flat and hollow white wares which are afore-described,referring to FIG. 5, FIG. 7 and FIG. 9.

Preparation of a gypsum slurry having carbon fibers uniformly dispersedtherein in a manner as described before, is somewhat troublesome in caseonly a very small amount of carbon fibers has to be exactly weighed atevery cycle of preparing the slurry. To avoid this, carbon fibers whichare light in weight, floating and hard to handle may be uniformlydispersed precedently in gypsum powder by utilizing either a circulatingjet air stream or a swinging and rotating movement. This premixing willmake it unnecessary to weigh a very small amount of carbon fibers forone batch at every cycle of preparing gypsum slurry.

The former method of mixing carbon fibers and gypsum powder by the useof a circulating jet air stream is particularly described following.Gypsum powder and carbon fibers being separated into single fibers arefirst charged into a mixer in a predetermined weight ratio. The mixtureis then carried by the jet air stream and circulated a number of timesthrough a closed line whereby the powder and the fibers are uniformlymixed. After the mixing, the mixture is collected from the air stream bymeans of a cyclone or bag filter to obtain a gypsum powder containingcarbon fibers uniformly dispersed therein. The jet air streat should becontrolled to have a pressure as low as 1 to 2 kgf/cm² (kg/cm²). Higherpnuematic pressures are not favorable since gypsum particles arecollided with one another or with carbon fibers at higher forces, sothat the gypsum particles and the carbon fibers are both reduced intofiner pieces, causing variation in physical properties as the gypsummold.

The latter method of mixing using the swinging and rotating movement isillustrated in FIG. 12, in which gypsum powder and single carbon fibersare charged into a flexible bag 20 in a predetermined ratio. The bag 20is swung and rotated through a swinging disc 21 attached at the bottomthereof. The disc has a swinging axle 23 concentrically mounted on arotary shaft 22. When the rotary shaft 22 is rotated, the bag is swungand rotated and thus are mixing in the bag 20 is accelerated. The speedand direction of movement are abruptly varied in the designed manner, sothat the gypsum powder and the carbon fibers are uniformly mixed. In theabove operations of mixing carbon fibers with gypsum powder, a hardeningretarder (curing retarder), a dehydrating agent (water reducer) andother necessary additives may be added simultaneously.

The gypsum mixture obtained above is a uniform mixture of carbon fibersseparated into a multitude of single fibers with gypsum powder. Ifcarbon fibers used are treated with a water-soluble sizing agent such aspolyvinylpyrrolidone, polyvinyl alcohol or the like, it does not neednecessarily that they are separated into single fibers. The reason whycarbon fibers may be admixed in the form of bundles with gypsum powderin the bag is that in preparation of a gypsum slurry, the water-solublesizing agents are dissolved and the carbon fibers are dispersedthemselves in water. Moreover, on each occasion of the above-mentionedthe higher content of carbon fibers than in use may be applied inadmixing with gypsum powder. This mixture having a higher content ofcarbon fibers shall be further admixed with fresh gypsum powder to givea predetermined ratio of gypsum powder to carbon fibers, preceding thepreparation of a gypsum slurry. According to this method, the cost oftransport for the gypsum powder materials may be saved in behalf of thecondensed carbon content.

When utilized as materials for general patterns, i.e. models andoriginal molds, the gypsum composition comprising carbon fibers has sosmall volume expansion upon hardening by hydration and so small volumeshrinkage in drying to succeed the hardening and removing the molds fromthe preceding mold such as a case mold in the case of forming mold, thatwhen the patterns of these molds are repeatedly duplicated by castinginto the preceding mold, i.e. parent mold (female mold), the resultingduplicated molds could have good accuracy. Moreover, the gypsumcomposition comprising carbon fibers has so small thermal expansioncoefficient that these models and molds less expand and less shrink evenif they suffer variation of temperature. Therefore, they could duplicatewith good accuracy and the models could be a good model of patternshaving high correctness.

Accordingly, repetition of the duplication several times gives goodresults because of the small difference in dimension between initial andfinal models or molds. This is the most important as a material forindustrial model or original mold.

On the occasion of making the gypsum models, besides the above describedcasting methods, such heaping up or laying up methods as a hand-lay upmethod in forming fiber-reinforced plastics could be also applied byusing the above described carbon fiber-dispersed gypsum slurry or moreviscous paste-like slurry. Moreover, regarding the industrial hugepatterns or models especially, it is preferable that they should havethe higher strength, and frequently they need not have such fineness asthe model or mold for manufacturing whiteware. Because of these reasons,the upper limitation in length of the chopped carbon fibers to bedispersed in a gypsum slurry can be 200 mm and the same in weight can be2.0 wt % based on hydrated gypsum. Nearby this upper limit, though theuniform dispersion of carbon fibers into gypsum slurry becomes moredifficult, the operation of forming or heaping up the models may becarried out effectively.

Examples of the present invention and Comparative Examples are describedfor the purpose of illustration only.

EXAMPLE 1

Polyacrylonitrile fibers were thermally treated at about 300° C. andfurther treated in an atmosphere of nitrogen gas at about 1300° C. forgraphitization thereby obtaining carbon fibers in bundles eachconsisting of about 600 single fibers with a diameter of about 7microns. The carbon fibers had physical properties of a tensile strengthof 300 kgf/mm² (kg/mm²), a tensile elastic modulus of 23,000 kgf/mm²(kg/mm²), a density of 1.75 g/cm³, a coefficient of linear thermalexpansion of -0.1×10⁻⁶ /°C., a thermal conductivity of 15 Kcal/m.hr.°C.(17.45 W/m.K), and a specific heat of 0.17 cal/g.°C. (0.71 kJ/kg·K). Thecarbon fibers were cut into pieces having a length of about 20 mm andwere separated into a multitude of single fibers in water by applicationof ultrasonic oscillation and agitation. One hundred parts by weight ofβ-gypsum powder, 60 parts by weight of water, 0.1 part by weight of thecarbon fibers and 0.2 part by weight of borax (hardening retarder) weremixed and agitated to obtain a gypsum slurry in which the single fiberswere uniformly dispersed. The gypsum slurry was cast into a case mold toobtain a gypsum forming mold for jiggering a flat white ware. The thusobtained gypsum mold was found to contain the single fibers uniformlydispersed in or throughout the cut section in the direction of diameter.The state of dispersing the carbon fibers was visually recognizable.

EXAMPLE 2

A slurry dispersion in which single carbon fibers were uniformlydispersed was prepared using the same conditions and procedure as inExample 1. A pure gypsum slurry free of any carbon fibers was separatelyprepared and cast into a case mold while rotating the mold at a lowspeed, thereby forming a thin layer of 1 to 3 mm thickness on the insidesurface of the case mold. Thereafter, the rotation of the case mold wasstopped and the carbon fiber-containing gypsum slurry was cast into thecase mold to obtain a gypsum forming mold for jiggering a white dishware of the flat ware type. The outer surface of this forming gypsummold which is a forming surface was covered with the thin film of puregypsum. No carbon fibers were exposed on the forming surface of thisforming mold.

EXAMPLE 3

Polyacrylonitrile fibers were treated with a heat treatment of about300° C. and then subjected to special heat treatment for graphitizationin an atmosphere of nitrogen gas at about 2500° C. Thus obtained singlefibers having a diameter of about 7 microns were treated with apolyvinylpyrrolidone sizing agent and combined to give bundles of carbonfibers each consisting of about 6000 single fibers. The carbon fibershad physical characteristics of a tensile strength of 250 kgf/mm²(kg/mm²), a tensile elastic modulus of 35,000 kgf/mm² (kg/mm²), adensity of 1.77 g/cm³, a coefficient of linear thermal expansion of-0.1×10⁻⁶ /°C., a thermal conductivity of 100 Kcal/m.hr.°C. (116 W/m.K),and a specific heat of 0.17 cal/g.°C. (0.71 kJ/kg.K). The bundles of thecarbon fibers were cut into pieces of 25 mm and weighed such that 0.3part by weight of the fibers per 100 parts by weight of gypsum powderwas used. The thus weighed carbon fibers were charged into a containerin which water was filled and were supplementarily agitated, whereuponthe carbon fiber bundles were separated themselves into a multitude ofsingle fibers and uniformly dispersed in water by agitation.Subsequently, β-gypsum powder, borax (hardening retarder) and MelmentF-20, (water reducer), made by Showa Denko K.K., were further chargedinto the dispersion and agitated to obtain a homogeneous gypsum slurryhaving such a composition of 100 parts by weight of gypsum powder, 60parts by weight of water, 0.3 part by weight of the carbon fibers, 0.2part by weight of the hardening retarder, and 0.2 part of the waterreducer. The gypsum slurry was cast into the case mold to obtain agypsum mold for slip casting adapted to form an elliptical dish of whiteware. The state of dispersion of the carbon fibers in the gypsum moldwas almost the same uniform dispersion as in Example 1.

COMPARATIVE EXAMPLE 1

One hundred parts by weight of β-gypsum powder, 60 parts by weight ofwater and 0.2 part by weight of borax were mixed and agitated therebyotaining a pure gypsum slurry free of any carbon fibers. The slurry wascast into a case mold to obtain a gypsum mold for jiggering a flat whiteware used to form a dish white ware.

Duplication of a model is described in the way of an example and acomparative example as follows.

EXAMPLE 4

Polyacrylonitrile fibers were treated with a heat treatment at about300° C. and then subjected to special heat treatment for graphitizationat about 1300° C. in an atmosphere of nitrogen gas. Thus single carbonfibers in the form of bundles each consisting of about 600 single fibershaving a diameter of about 7 microns were obtained. The carbon fibershad physical characterristics of a tensile strength of 300 kgf/mm²(kg/mm²), a tensile elastic modulus of 23,000 kgf/mm² (kg/mm²), adensity of 1.75 g/cm³, a coefficient of linear thermal expansion of×0.1×10⁻⁶ /°C., a thermal conductivity of 15 Kcal/m.hr.°C. (17.45W/m.K), and a specific heat of 0.17 cal/g.°C. (0.71 kj/kg·K). The carbonfibers were cut into pieces having a length of about 20 mm and separatedinto a multitude of single fibers in water by the synergistic action ofultrasonic oscillation and mechanical agitation. One hundred parts byweight of β-gypsum powder, 60 parts by weight of water, 0.1 part byweight of carbon fibers and 0.2 part by weight of borax (hardeningretarder) were mixed together and agitated to obtain a gypsum slurry inwhich the single carbon fibers were uniformly dispersed. This gypsumslurry was cast into a gypsum female mold designed to duplicate aturbine blade of a gas turbine, thereby obtaining the duplicated malegypsum model. This gypsum model contained the single carbon fibersuniformly dispersed in or throughout the model including the cutsectional portions. The state of dispersion of the single carbon fiberswas visually recognizable.

COMPARATIVE EXAMPLE 2

One hundred parts by weight of β-gypsum powder, 60 parts by weight ofwater and 0.2 part by weight of borax were mixed and agitated to obtaina pure gypsum slurry free of any carbon fibers. The thus obtained gypsumslurry was cast into a gypsum female mold designed to duplicate aturbine blade of a gas turbine, obtaining the duplicated male gypsummodel.

The duplication accuracy between the female mold and the released malegypsum molds of Example 4 or Comparative Example 2, and the variationratio at different temperatures are as follows.

As regards the duplication accuracy after releasing the model, theconventional product having no carbon fibers was found to be shrunk by0.066 mm relative to 500 mm, whereas the inventive product was shrunk byas small as 0.018 mm to 500 mm. The variation ratio in length was asfollows: the conventional product was expanded by 0.075% at the maximumexothermic temperature (53.2° C.), whereas the product of the inventionwas expanded by 0.066%. At room temperature (23.5° C.), the expansion ofthe conventional product 0.025%, whereas the expansion of the product ofthe invention was 0.018%. As will be clear from the above result, theproduct of the invention is less expanded after the hardening byhydration with the resulting duplication accuracy being good.

The gypsum molds obtained in Examples 1, 2 and 3 and Comparative Example1 have physical characteristics including bending strength, waterabsorbability, temperature difference to rupture in air, bulk specificgravity and expansion ratio at the end of hardening, as shown in Table 1below.

    ______________________________________                                                      Examples Example  Comp.                                                       1, 2     3        Ex. 1                                         ______________________________________                                        Bending strength (kgf/cm.sup.2)                                                               95         100      60                                        Water absorbability (%)                                                                       24.5       24.5     22                                        Temperature difference to                                                                     60-65      70-75    40-45                                     rupture (°C.)                                                          Bulk specific gravity                                                                         1.12       1.12     1.17                                      Expansion ratio at the                                                                        0.12       0.12     0.17                                      end of hardening                                                              ______________________________________                                    

The term "water absorbability" means a weight percentage of the maximumabsorbed water to the weight of the wet test piece after a dried testpiece was immersed in water at normal temperature and normal atmosphericpressure.

The term "temperature difference to rupture" means a minimum temperaturedifference between a heated temperature and room temperature, in case atest piece has broken down due to thermal stress exerted thereon whenthe test piece was heated to a given temperature in air and immediatelyafter placed in a room for permitting the piece to stand at roomtemperature in atmospheric air.

The term "expansion ratio at the end of hardening" means a percentage ofthe variation in length of the casted gypsum mold to the preceedingmold, i.e. parent mold before drying and after hardening process whereinitially the casting becomes itself warm about 50° C. due to theexothermic reaction within about one hour, then it is laid at roomtemperature to be cooled till normal temperature, in the gypsum slurrycasting and succeeding curing process. The above whole casting andsucceeding curing process consists of casting a gypsum slurry into thepreceeding mold, succeeding hardning accompanying with exothermicreaction, removing the casting from the preceeding mold after cooling,and drying or perfect curing for about three to seven days at 50° C. ina hot air dryer.

As will become apparent from the above table, the gypsum moldscontaining carbon fibers therein have much improved the bending strengthand the temperature difference to rupture in air over the carbonfiber-free mold. The other physical characteristics are also improved.

FIG. 14 is a two thousand multiplied picture took by the scanning typeelectron microscope from a replica of the cutting section in the gypsumforming mold of Example 1. The straight circular bar elongated fromupper lefthand toward central portion is a single carbon fiber admixedin, and crystalline blocks as broken pieces are crystals of gypsumdehydrate produced from β-hemihydrate gypsum.

FIG. 15 is also a two thousand multiplied picture took by the samescanning type electron microscope from the same as in Example 1, i.e.picture 1 except using α-hemihydrate gypsum powder.

FIG. 13 shows the relation between bending force and elapsed time havingmade the bending tests the carbon fibers content as a parameter whenusing 15 mm×25 mm×250 mm gypsum composition test pieces each piece wasbended by a bending tester under conditions indicated below. The gypsumtest pieces are made from slurries of 100 parts by weight of β-gypsum,60 parts by weight of water and different amounts of pitch carbon fiberswith a length of 25 mm, succeeding by casting and drying to obtain acured test piece. As will be apparent from FIG. 13, the higher contentof the carbon fibers result in the greater bending force, solving theproblem of simple rupture of the piece at the maximum bending force andensuring toughness as a composition material. It will be noted that thetest equipment had a span of 200 mm, a load was applied at the center ofthe test piece, and a displacement speed of the loading point was 1mm/min.

FIG. 16 shows curves of a expanding variation percentage in lengthversus temperature for two different types of test pieces, a pure gypsumcomposition text piece and a carbon fiber-reinforced gypsum compositiontest piece, respectively. The variation ratio in length of the testpiece of gypsum composition admixed with carbon fibers is slightlysmaller as in each case of positive or negative expansion, in absolutevalue, than a variation ratio of the gypsum composition test piece freeof any carbon fibers. Presumably, this is because carbon fibers whosethermal expansion coefficient is almost zero enter inbetween particlesof gypsum and thus serve to suppress the expansion or shrinkage (ornegative expansion) of gypsum. Accordingly, gypsum molds made of amixture of gypsum and carbon fibers suffer thermal expansion orshrinkage only with a small degree in temperature variation ofcircumstance, therefore molded articles by these molds should haveimproved dimensional accuracy.

Because bending strength or resistive bending force are remarkablyimproved, the strength of material mechanics of the gypsum moldincreases. Hitherto, known gypsum forming moldes are often broken byexternal pressure or mechanical force imposed to them in the process offorming. The portions of a gypsum forming mold which tended to be brokenhave increased strength, according to the invention, in mechanicalstrength, by which breakage of the carbon fiber-reinforced gypsum moldsis prevented and a short cycle time of forming operation is possible,resulting an improvement of forming ability (productivity) per mold anda prolonged life of the gypsum forming mold. Breakage of a gypsum moldduring forming may damage a forming machine and also interrupt theforming operation. This is suitably prevented by the use of the carbonfiber-reinforced gypsum forming mold. In addition, time required forre-adjustment after exchange of damaged parts of the forming machine orremoving the broken gypsum mold and so on may also be saved. Muchimproved mechanical strength enables one to render a thickness of agypsum mold small, reducing an amount of gypsum to be used.

The reason why the temperature difference to rupture in air is improvedby admixutre of carbon fibers is considered as follows. The gypsumparticle itself is expanding to a certain extent by temperature rise butthe chopped single fiber of carbon fiber is little expanding. The carbonfibers in the structure of a gypsum mold is applied with a tensile forcealong the length thereof and the gypsum particles are compressed. As aresult, internal stress is imposed on the carbon fibers along theirlength, thereby causing the state of introducing such prestress as aprestressed concrete, i.e. PS concrete.

A pronounced increase in the temperature difference to rupture in airmeans, especially for manufacturing white ware, that a gypsum mold iscapable of using under the greater temperature difference between thedrying temperature and the temperature of forming circumstance. For theforming, casting or jiggering, of white wares, a gypsum mold afterforming can be dried at elevated temperatures. Accordingly, it ispossible to shorten the drying time for each damped gypsum mold. Anumber of molds to be retained in a dryer are reduced resulting in animprovement of forming efficiency (productivity). This leads to the factthat a number of working gypsum molds to a number of formed white warescan be reduced. This fact, that is to say, use of the carbonfiber-reinforced gypsum, is suitable for manufacturing articles ofvarious types in reduced amounts and can reduce the prime cost of finalproducts.

In the case of jiggering white ware with the gypsum molds, in behalf ofthe improvement of the water absorbability, the time requiring from thecompletion of forming till the removal of the green ware from the moldis shortened. With the gypsum molds for slip casting of white wares, inbehalf of this improvement, the time requiring for growing a green filmfrom a casting slip is shortened. In both cases, the number of gypsummolds being retained in the forming and succeeding drying process arereduced with the forming efficiency being improved.

The lowering of the bulk specific gravity allows gypsum molds to belight in weight and thus contributes to improve ability of transportingand handling of gypsum molds. By only a slight reduction of the volumeexpansion ratio at the end of hardening, an expansion pressure exertedon case mold is reduced at the time of forming the gypsum forming moldfor white wares, permitting the forming mold to be readily removed andpreventing breakage of the case mold.

The above described advantages of the reduced volume expansion is alsoapparent in the case of duplicating model or any other mold than thewhite ware forming mold.

In view of the foregoing, the effects of the present invention may besummarized as follows.

(1) Strength of material mechanics of the gypsum mold is much improved.Breakage of the gypsum mold by external pressure exerted at theoperation of forming is prevented and strength (i.e. durability) againstthe internal stress caused by a certain temperature difference isnoticeably increased. For the forming of white wares or any otherforming to use absorbability of the gypsum mold, the drying temperatureof the damp gypsum mold can be raised, leading to reducing the dryingtime. Therefore, complete drying is possible resulting in remarkablyincreasing the forming efficiency.

(2) Carbon fibers having a very small diameter but large strength andhigh flexibility are used as a reinforcing material for gypsum mold.Such carbon fiber reinforced gypsum can increase strength againstmechanical external force and uneven temperature distribution eventhough the mixing ratio of the fibers to gypsum is small. Because of thesmall mixing ratio of the reinforcing material, the water absorbabilitywhich is one of fundamental functions of gypsum mold for forming whiteware, paper ware and so on by using absorbability does not lower byadmixing the carbon fibers.

(3) When the surface layer portion of gypsum mold which is a formingsurface covered with a thin layer or film of pure gypsum, it iscompletely prevented that the ends of carbon fibers are exposed on theforming surface, ensuring the resultant mold having a smooth surface.

(4) Carbon fibers which burn out by heating are used as a reinforcement.When the gypsum models or molds are broken in use or does not stand use,carbon fibers contained in the mold composition may be readily removedalone by crushing the models and molds, and succeeding gently heatingthe crushings taking a long time and the resultant gypsum crushings orhemihydrate gypsum may be reused. In this regard, if a reinforcingmaterial which is not burnt out or not removed by heating such as glassfibers is used, it is very difficult or rather impossible to remove theadmixed reinforcing material alone for regeneration or reuse of thegypsum models or molds.

(5) After cutting the carbon fibers into a predetermined length, asizing agent deposited thereon is decomposed and removed by heating andthe carbon fibers are agitated in water by gently rotating agitatorblades applying ultrasonic oscillation, by which the bundles of veryflexible carbon fibers with a very small diameter can be separated intoa multitude of single fibers. By succeeding water separation, drying andadmixing the prepared materials, the resulting gypsum slurry has amultitude of single carbon fibers dispersed uniformly therein and thusthe carbon fibers are uniformly distributed in a gypsum model or mold.

(6) In case where carbon fibers treated with water-soluble sizing agentsare used, they are charged into water, after cutting into apredetermined length, by which the water-soluble sizing agent is causedto be dissolved out in water. Therefore, the bundles of carbon fibersare instantly separated into a multitude of single fibers and dispersedinto water by gentle agitating. The gypsum powder is charged into thedispersion and agitated thereby obtaining a gypsum slurry in which thesingle carbon fibers are uniformly dispersed. In this case, anypretreatment for separating bundles of carbon fibers into a multitude ofsingle fibers is unnecessary.

(7) Carbon fibers are first cut into a predetermined length andseparated preparatively into a multitude of single fibers. Gypsum powderand a predetermined amount of the single carbon fibers relative to thegypsum powder are charged in a circulation of jet air stream touniformly premix them together. The mixture is then collected and air isseparated to obtain a large amount of the powder mixture having apredetermined mixing ratio. This makes it unnecessary to weigh carbonfibers only in a small amount required for one batch in order to preparea gypsum slurry.

(8) Conventionally, for the purpose of forming a fiber-reinforcedcomposition, casting of a gypsum slurry into a preceeding mold iscarried out as follows. A gypsum slurry is first cast into thepreceeding mold, i.e. the mother mold to form a first surface layer witha thickness of 5 to 10 mm. On the first layer, a second layer is formedin the same manner of casting. Then such a chopped reinforcing fibermaterials as hemp are scattered over the second cast layer and forcedthem into the cast layer of a gypsum slurry by fingers. As a result, thereinforcing fiber material are incorporated with the gypsum slurry inthe cast layer. Subsequently, third and fourth layers are heaped upsuccessively in the same manner as described the above to apredetermined total thickness.

In contrast with this, in the practice of the present invention, thecarbon fiber reinforcing materials are dispersed preparatively in agypsum slurry. The agitated carbon fiber premixed slurry is cast to forma relatively thick first layer, followed by merely casting the agitatedslurry to form a second layer to a predetermined thickness. In otherwords, according to the present invention, a large amount of the slurrycan be cast into the mold at a time, enabling the casting and workingtime to be shortened.

(9) The carbon fiber dispersed gypsum of the present invention has sucha small volume expansion to the preceding mold at the time of hardeningby hydration that in the manufacture of molds for forming white wares,errors are small in the duplication from an original mold, via a basicmold and a case mold to a forming mold. The resultant forming mold withhigh accuracy is obtained. The other forming mold, e.g., the mold for adie casting, a pulp slurry casting, a synthetic plastics injection, afine ceramics isostatic pressing and so on, also can be duplicated inhigh accuracy.

(10) In manufacturing metal molds for casting metal materials such as "aprecision metal casting", if the present invention of the carbon fibersdispersed gypsum composition is used instead of the metal molds that area master mold, i.e. a mold duplicated the model (corresponding to abasic mold in manufacturing white ware), a preceding mother mold for theforming mold which is duplicated from the master mold (corresponding toa case mold in manufacturing white ware) and a forming mold, these 3types of molds could be produced easily and precisely. Consequently, theproducts should be manufactured easily or instantly and precisely.

Namely, casted selling samples can be manufactured in extreme easinessand in high accuracy. Moreover, the application of the carbon fiberdispersed gypsum can be answered instantly and economically againstchange of the design thereafter, too.

Besides, comparing with a sand mold or a metal mold, the application ofthe present invention could assure a higher accuracy than each of thesemolds, and manufacture a fewer lots of products. Especially, the presentinvention is applicable to the production of a more variety and a fewerproduction such as die-casting moulding.

(11) In duplicating ordinary industrial model (original mold)repeatedly, it is possible to manufacture the duplicated original moldof an extremely small error in such high accuracy and fineness that evena fingerprint and a wood grain could be duplicated.

Comparing to a pure gypsum or a carbon fiberless gypsum composition, theinvention of a carbon fibers dispersed gypsum composition provides thesame less expansion in hardening and increased strength as described thepreceding (9), so that a large or huge industrial model can beduplicated in high accuracy.

(12) Though, in the case of a complicated pattern such as containingengraving patterns of a deep under cutting pattern, it is necessary forthe said portion of the complicated pattern in such an intermediary moldas a basic (master) mold and a case mold that the said mold surface isconstituted by such a flexible material as silicone synthetic rubber,the present invention applying to the original mold or the forming moldshould assure to be produced more extremely easily and more preciselythan the some of a carbon fiberless gypsum composition.

(13) Carbon fibers have so soft and flexible that even though they arecontained in a gypsum model or original mold, the surface is smooth witha suitable degree of softness. Accordingly, machinability of the modelor original mold is good.

(14) Even in such a complicated pattern as containing an undercut, themodel and the original mold of the invention are machined and engravedfar and away more easily than a metallic or a wooden model and originalmold. The resultant gypsum model or original mold are also duplicatedeasily and precisely, too. For instance, even such a fine pattern as afingerprint or wooden grain could be duplicated faithfully andcorrectly.

Consequently, if only a little volume expansion (±0.1%) in hardening byhydration is allowed, the duplicated model could be used as a model,too. Thereby, in the case of wooden model reproducing cost of woodenmodel sample ordinarily used is saved.

Considering the viewpoint of producing a model itself instantly andeconomically, the invention is most preferable to production of themultiplex variety and the fewer products.

What is claimed is:
 1. A carbon fiber-reinforced gypsum model comprisinga gypsum structure of a matrix, said structure of a matrix comprisinggypsum and from 5 to 200 mm long carbon fibers uniformly dispersedtherein in the form of single fibers in an amount rainging from 0.008 to2.0 wt % based on said gypsum.
 2. A carbon fiber-reinforced gypsum modelaccording to claim 1, wherein from 5 to 100 mm long carbon fibers isuniformly dispersed in the gypsum structure of a matrix in an amount offrom 0.008 to 0.9 wt % based on said gypsum.
 3. A carbonfiber-reinforced gypsum forming mold or its preceding molds, comprisinga gypsum structure of a matrix, said structure of a matrix comprisinggypsum and from 5 to 100 mm long carbon fibers uniformly dispersedtherein in the form of single fibers in an amount ranging from 0.008 to0.9 wt % based on said gypsum.
 4. A carbon fiber-reinforced gypsumforming mold or its preceding molds according to claim 3, wherein saidforming mold is a slip casting mold, a press forming mold, an extrusionforming mold, or an injection forming mold.
 5. A carbon fiber-reinforcedgypsum forming mold or its preceding molds according to any one ofclaims 3 through 4, wherein from 15 to 30 mm long carbon fibers areuniformly dispersed in the gypsum structure of a matrix in an amount of0.05 to 0.3 wt % based on the gypsum.
 6. A fiber-reinforced gypsumforming mold or its preceding molds comprising:(a) being applied to aforming method by using raw materials for pottery, porcelain orrefractories, fine ceramics materials which are free from clay minerals,the other plastic raw materials or a pulp slurry for manufacturing paperwares; (b) being composed of a gypsum structure of a matrix, saidstructure of a matrix comprising gypsum and from 5 to 100 mm longreinforcing fibers uniformly dispersed therein in the form of singlefibers in amount ranging from 0.008 to 0.9 wt % based on said gypsum. 7.A fiber-reinforced gypsum forming mold or its preceding molds, accordingto claim 6, wherein said forming mold is a slip casting mold, a pressforming mold, an extrusion forming mold, or an injection forming mold.8. A fiber-reinforced gypsum forming mold or its preceding molds,according to claim 6, wherein said forming mold is a forming mold forjiggering pottery or porcelain white wares.
 9. A fiber-reinforced gypsumforming mold or its preceding molds, according to any one of claims 6 toclaim 8, wherein said fiber is a carbon fiber.
 10. A carbonfiber-reinforced gypsum model comprising a gypsum structure of a matrixsaid gypsum structure of a matrix comprising gypsum and from 5 to 200 mmlong carbon fibers uniformly dispersed therein in the form of singlefibers in an amount ranging from 0.008 to 2.0 wt % based on said gypsum,an external surface of said gypsum mold being covered with gypsum ofless content of carbon fibers comprising with the content of carbonfibers in inner portion.
 11. A carbon fiber-reinforced gypsum modelaccording to claim 10, wherein from 5 to 100 mm long carbon fibers areuniformly dispersed in the gypsum of the matrix body in an amount offrom 0.008 to 0.9 wt % based on the gypsum.
 12. A carbonfiber-reinforced gypsum mold or its preceding molds for forming whitewares comprising a gypsum structure of a matrix, said structure ofmatrix comprising gypsum and from 5 to 100 mm long carbon fibersuniformly dispersed therein in the form of single fibers in an amountranging from 0.008 to 0.9 wt % based on said gypsum, a forming surfaceof said gypsum mold being covered with pure gypsum which is free fromcarbon fibers.
 13. A carbon fiber-reinforced gypsum mold or itspreceding molds, according to claim 12, wherein from 15 to 30 mm longcarbon fibers are uniformly dispersed in the gypsum of the matrix in anamount of from 0.05 to 0.3 wt % based on the gypsum.